It is paramount for drug delivery devices to possess the ability to dose accurately in order to ensure that a user receives the intended treatment. Injection devices which comprise a cartridge filled with a medicament and closed with a movable piston, and a dose delivery mechanism including a piston rod for moving the piston are dependent on a precise transfer of the movement of the piston rod to the piston in order to achieve the required dosing accuracy. Any flexibility in a dosing system will affect the dosing accuracy because flexibility will introduce a delay between the movement of the dose delivery mechanism and the piston. If the delay is too large there is a risk that the user will end the injection procedure (e.g. remove the injection needle from the skin) before the intended dose is actually received.
An important contributor to flexibility in the dosing system is the potential presence of air in the medicament. An air bubble in the cartridge will act as a spring and introduce a significant delay to the system. The larger the air bubble the larger the delay that is introduced.
A common misuse of injection devices consists in the user not removing the needle from the needle holding part of the injection unit after an injection. If the needle is not removed air can enter the cartridge through the needle lumen and introduce an uncontrolled delay to the dosing system, thereby compromising the dosing accuracy of the injection device. In order to mitigate this situation users are recommended to prime the device, i.e. to perform an air shot, prior to each injection in order to remove excess air from the cartridge. However, priming is considered a hassle and is sometimes neglected. The usability of injection devices would therefore be significantly improved if the need for air shots could be eliminated.
One way of avoiding air entry into the cartridge through an attached needle is to ensure that fluid communication between the needle and the cartridge is interrupted immediately after an injection. In WO 2011/051366 (Sanofi-Aventis Deutschland GmbH) this is done by retracting the cartridge in the cartridge holder as soon as a dose dispensing procedure has terminated, whereby the needle is pulled out of the cartridge septum. However, by this action the cartridge is displaced relative to the drug delivery device housing, which holds the drive mechanism. The next dose dispensing procedure requires the drive mechanism to firstly displace the cartridge back into fluid communication with the needle before the cartridge piston can be moved to actually dispense a dose. In a precision delivery device the movement of the piston actuator must be closely correlated with the movement of the piston relative to the drug container wall to ensure that the correct dose is expelled. For such delivery devices it is preferable that all advancing movements of the piston actuator are converted directly to advancements of the piston in the drug container because dose delivery is based on a well-defined displacement of the piston actuator relative to the housing. It would be undesirable to additionally take account of a part of the piston actuator movement being used to advance the drug container itself.